Encoders are usually classified as rotary encoders and linear encoders, and rotary encoders are further classified as incremental encoders and absolute encoders. In particular, absolute encoders include multiple rotation-type encoders which detect the number of rotations. As described, for example, in Japanese Published Patent Application No. 6-41853, some multiple rotation encoders have mechanisms for detecting the displacement and angle of displacement within a single rotation, and some have mechanisms for detecting the number of rotations in more than one rotation.
The multiple rotation encoder described in Japanese Published Patent Application No. 6-41853 is equipped with an optical absolute value encoder for detecting the absolute angle within a single rotation, and a magnetic encoder for detecting multiple rotations. The optical absolute value encoder for detecting the absolute angle within a single rotation includes a rotating disk attached to the shaft for detecting the absolute angle within a single rotation, an LED for projecting light to this disk, a light receiving element photodiode array for receiving light from this LED through a stationary slit, and a waveform shaping circuit for shaping the waveforms of the detected signals from this photodiode array, among other parts. The magnetic encoder for detecting multiple rotations includes a rotating disk equipped with a magnet (a so-called “ring magnet”) on the rotating part, a magnetic resistance element for detecting the rotation of this rotating disk, a waveform shaping circuit for shaping the waveforms of the signals from this magnetic resistance element, and a control circuit (counter) for counting the detected signals of multiple rotations and storing numerical values, among other parts.
Because magnetic poles reverse during one rotation, the rotation of the ring magnet can be detected by the magnetic resistance element by detecting change in these magnetic poles. Moreover, the mechanisms of such an encoder for detecting a single rotation and multiple rotations may be either optical or magnetic mechanisms, and such aspects as the structure and attachment location of their parts have various arrangements depending on the type of encoder.
As described, for example, in Japanese Published Patent Application No. 7-218290, only an optical system may be used for the detection mechanism and an optical system may be used to generate the signals used for detecting multiple rotations.
As described in Japanese Published Patent Application No. 10-325740, only an optical system may be used for the detection mechanism and the high-order bits of absolute signals may be used for the signals used to detect multiple rotations. Encoders where the multiple rotation detection part is magnetic are usually considered better because they may run with less power and may provide for prolonging the lifetime of the backup power.
Whichever system is used, the fundamental signals used to detect multiple rotations, as illustrated in FIG. 6, usually include Phase A and Phase B signals which differ in electrical angle by a 90° phase and complete one cycle per single rotation. Using signals with such a phase difference makes it possible to detect the direction of rotation and count the number of rotations according to the direction of rotation. Thus, FIG. 6 is a timing chart showing the state of each fundamental signal used to detect multiple rotations.
Because one count per single rotation may be used when using such fundamental signals to count the number of rotations, the change points for either the Phase A or Phase B signal is normally used to count the number of rotations. FIG. 7 is a timing chart using the Phase A and Phase B signals and the three low-order bit outputs Count (0) to Count (2) outputted by the counter to show the operation of such multiple rotation counting. In this example, a rotation is counted by detecting the rising edge of the Phase A signal. If the direction of rotation is clockwise, the count outputs Count (0) to Count (2) of the counter are incremented at each rising edge of the Phase A signal, and the number of rotations is counted as “0, 1, 2, 3. . . ”
However, if an occurrence such as pulse breakup due to noise or oscillation makes it impossible to detect the rising edge of Phase A, this produces a count error as illustrated in FIG. 8, but inability to recognize count errors from the states of the Phase A and Phase B signals and the count outputs Count (0) to Count (2) may lead to overlooking count errors. Thus, FIG. 8 is a timing chart illustrating the state when a detection error (NG) occurs at an edge of Phase A in FIG. 7.
More precisely, such problems occur because when the levels of the Phase A and Phase B signals change as shown in Table 1, the corresponding states of the count output Count (0) can be both 0 and 1, and the correlation between these make it impossible to detect count errors.
TABLE 1Phase APhase BCount (0)000/1100/1010/1110/1
When such a detection error occurs in the multiple rotation part of a multiple rotation encoder, it constitutes a single rotation error, which is an error the extremely high figure for position control data of which runs the risk of creating fatal problems.
Although the same manner of counting using the Phase A and Phase B signals is also used by these incremental encoders and absolute encoders to detect displacement in a single rotation, this case also risks generating count errors in the same manner. Consequently, such count errors must be prevented to improve the reliability of encoders.